NR121E Datasheet, PDF (7/12 Pages) Sanken electric – Buck Regulator

3A, 18V, Buck Regulator
NR120E series
May/16/2012
Typical Application Circuit
V_IN
V_IN_s
C1
C2
GND
R3
R1
2
IN
7 _EN
1
C10
BS _SW 3
L1
NR121E
R5
8 _SS
GND
C9
4
FB ⁵
NC
R4
6
R6
D1
SW
Vo
Vo _s
C4 C5
GND
C1, C2: 10μF / 25V
C4, C5: 22μF / 16V
C9: 0.1μF
C10: 0.1μF
R1: 100kΩ
R3: 22Ω
R4: 8.2 kΩ, R5: 4.3kΩ (V_O=3.3V)
R6: 3.9kΩ
D1: SJPB-L4
L1: 10μH
External Components Design Guide
(1)Diode D1
・The schottky-barrier diode must be used for D1. If other diodes like fast recovery diodes are used, IC may be
damaged because of the reverse voltage applied by the recovery voltage or ON voltage.
(2)Choke coil L1
・If the winding resistance of the choke coil is too high, the efficiency may go down to the extent that it is out of the rating.
・As the start current of the over current protection is approximately 4A, attention must be paid to the heating of the choke
coil by the magnetic saturation due to overload or short-circulated load.
(3)Capacitor C1(C2), C4(C5), C9
・As large ripple currents across C1 (C2) and C4 (C5), capacitors with high frequency and low impedance for SMPS must
be used. Especially when the impedance of C4 (C5) is high, the switching waveform may not be normal at low
temperature.
・C9 is a capacitor for soft start. In case soft start function is not used, please keep No.2 Pin open.
(4)Resistor R4, R5, R6
・R4, R5, R6 are resistor to the Output Voltage. I_ADJ is required to set to 0.2mA.
R4, R5, R6 are calculated by the equation (1).
R4  R5  ^{Vo  VFB }  ^{Vo  0.8} ， R6  ^VFB  ^0.8 ≒ 4.0k
I _ADJ
0.2 10^3
IADJ _{0.2 10}3
------- (1)
In order to have optimum operating condition, each component must be connected with the minimum distance.
Rev.0.3
SANKEN ELECTRIC CO., LTD.
7
http://www.sanken-ele.co.jp